The prior art, as exemplified in U.S. Pat. Nos. 4,114,063 and 4,131,815 and a publication by John P. Downing, "Particle Counter for Sediment Transport Studies," Journal of Hydraulics Division, Proceedings of the American Society of Civil Engineers, ASCE, Vol. 107, No. HY11, Nov. 1981, pages 1455-1465, contains sensors for measuring the flux of moving solid particles by sensing the vibrations produced when the particles impinge upon a stationary target. The vibrations produced by such particle impacts, pings, are damped mechanical oscillations which are converted to electrical signals by an electromechanical transducer, such as ceramic piezoelectric elements, coupled to the target. The transducer signals are amplified, filtered and detected to produce pulses which are counted over predetermined time periods. These counts which correspond to the particle flux can be transmitted or recorded for later recovery. The above U.S. Pat. No. 4,114,063 uses a bandpass range from 10 kHz to 1 mHz; the above Downing publication uses a much narrower bandpass centered at 100 kHz; and the U.S. Pat. No. 4,131,815 uses a resonant frequency in the range of 700 kHz.
The ping rates or counts of detected oscillation events provide a direct in situ measurement of particle-mass flux when the effective cross section (ECD) of the target, the counting efficiency (CE), mean particle diameter, and particle density are known The ECD is the cross-sectional area, including the sensor cross-section, over which a particle has a high probability, say 0.95, of being detected if it passes through that area. The ECD is usually larger than the cross-sectional area of the sensor because grazing impacts can produce detectable pings. Counting efficiency equals (counts detected/actual impacts) .times.100%. Because ECD and CE are difficult to calculate from basic principles of shock and vibration, they are rarely determined explicitly. Instead, a calibration factor is empirically established for relating ping rates to particle-number flux. The calibration factor implicitly contains the product, ECD.times.CE. When it is multiplied by the ping rate, in counts per second, the particle-number flux, in particles per unit area per unit time is obtained.
One major problem in applications of particle sensors is the abrasive, acoustically noisy, and destructive nature of the media being sensed. Large particles, whether transported in fluids or by mechanical conveyors, have huge destructive action resulting from shock loads. Other causes of damage which shorten the mean time to failure of sensors include, persistent low-energy vibration and fatigue, corrosion by fluids, and bending loads imparted by very large objects, such as trees carried by flooding rivers or streams. To survive in hostile environments, a sensor must be armored and constructed to sustain very large loads without failure.
Another problem concerns collecting and retaining data from the particle sensor. Prior art techniques of connecting sensors to cables, to telemetry apparatus, or to recording devices, are generally unsuitable for hostile environments such as streams where conditions can readily cause failure of the cabling.
In monitoring gravel transport in streams, it is rarely convenient, and frequently impossible, for an operator to be present at all times when measurements are required. Rapid changes in stream condition, such as caused by storms and high water, can produce major changes in the geometry of a gravel stream bed. Point bars can be moved several tens of meters in a few hours, burying a sensor in a low-lying portion of the stream in gravel deposits. A sensor can be displaced tens of meters downstream. Finding the location of a buried and/or displaced sensor can be difficult.